The aim of the proposed project is to further define the natural history of nephrocalcinosis (calcifications within kidney tissue) in the premature infant and to identify factors which would distinguish infants most at risk. Some infants who develop nephrocalcinosis resolve spontaneously, without treatment, while others have persistent calcium deposits in the kidney which do not resolve. Currently, there is no way to predict which infants will have persistent nephrocalcinosis, and it is not clear whether this persistent nephrocalcinosis is of any clinical significance. Preliminary studies showed that a markedly elevated urinary calcium-to- creatinine ratio may be predictive of which patients will have persistent nephrocalcinosis. The current project proposes to screen a specific group of premature infants most at risk for developing nephrocalcinosis and follow them for two years after birth. The work proposed in this study will allow children at risk to be identified much earlier in their lives, with the hopes that earlier identification will allow more aggressive treatment and follow-up. This work will also better define the premature infant population which does not develop nephrocalcinosis, giving future investigators a much better control group reference.